For a variety of consumer and commercial applications, electronic equipment requires that power be provisioned through alternating current (AC) power lines, such as the standard 60 Hz power lines in the United States. These AC power lines are typically referenced to earth ground. Power supplies are employed to convert input AC voltages to desired level(s) of direct current (DC) voltage needed by the internal components of the electronic equipment. Equipment often requires DC isolation between portions of their resident circuitry and earth ground for proper circuit operation and user safety, and power supplies generally provide such isolation.
Ideally, an isolated power supply should accept a wide range of AC power line voltages while producing a stable, constant DC voltage. Preferably, this capability is performed with essentially no power loss, and without producing any undesirable side effects that adversely impact the performance of the associated equipment. One such side effect commonly produced by power supplies is injected current, also referred to as common-mode AC current, that is induced in the DC outputs of a power supply due to coupling between the isolated and non-isolated portions of the supply. Typically, inter-winding capacitance in a power transformer within the power supply provides the major source of such coupling. High frequency applications, in particular, present problems in the design of transformers with reduced winding capacitance.
The injected current produced depends to some degree on the magnitude of the inter-winding capacitance and the magnitude, primary frequency, and harmonic content of the driving voltage applied at the terminals of the primary winding of the transformer. Winding capacitance can be separated into four categories: (1) capacitance between turns; (2) capacitance between layers; (3) capacitance between windings; and (4) stray capacitance, i.e., capacitance between the winding next to the core and the outer winding next to the adjacent circuitry.
A traditional isolated power supply 100, as shown in FIG. 1, normally exhibits some level of injected current. An AC line voltage VACL, drives the primary windings of a power isolation transformer 110, thereby inducing a smaller AC voltage VACT in the transformer secondary windings. This voltage then drives a rectifier 120, which converts the smaller AC voltage VACT into a half-wave or full-wave rectified voltage VACR. A low-pass filter 130 then converts the rectified voltage VACR into a DC voltage VDCR exhibiting some small AC “ripple.” In some cases, a voltage regulator 140 is then employed to generate a DC output voltage VDCO with reduced AC ripple, while also providing some DC voltage level stability against amplitude changes in the AC line voltage VACL.
Usually, the rather high amplitude of the AC line voltage VACL produces a high level of injected current by way of the aforementioned inter-winding capacitance of the power transformer 110. Capacitance between the layers of the primary and secondary windings of the transformer contribute most to the overall capacitance, while winding-to-winding capacitance and stray capacitance perhaps contribute most to common-mode noise and circuit instability. In order to mitigate against this, special winding techniques, such as balanced windings, and electrostatic shielding are employed to reduce the capacitance, but at the expense of a more costly transformer.
In addition to the above, the traditional power supply 100 normally exhibits low power efficiency. This characteristic is caused by a relatively low transformer primary-to-secondary winding ratio to ensure that the smaller AC voltage VACT is of a sufficiently high magnitude when the AC line voltage VACT, is at a minimum. Therefore, when the AC line voltage VACT, is higher than its minimum, the smaller AC voltage VACT is higher than necessary to provide the required DC output voltage VDCO, resulting in higher power dissipation, sometimes resulting in power efficiency as low as 50%, or less.
Furthermore, in order to produce the same DC voltage for a wide range of AC line voltages VACL, the transformer 110 often employs multiple windings and taps to allow different AC line voltages VACL, to produce the same DC output voltage VDCO. The tap to be used in a particular circumstance is selected by way of a user-settable switch. While such a design lends flexibility with respect to the various AC power line voltages with which the associated equipment may be used, increased hardware costs and possible equipment damage result due to an incorrect switch setting.
To address this drawback, a switch-mode DC-to-DC converter 200, as displayed in FIG. 2, may be employed as part of a larger power supply. First, an AC power line voltage (not shown) is converted to a relatively high DC input voltage VDCI by way of rectifiers and filter capacitors (also not shown), typically by way of a line voltage selector switch. The DC input voltage VDCI is then quickly switched ON and OFF intermittently via an electronic switch circuit 210 (typically comprising a set of transistors), resulting in a switched DC voltage VDCS. The switched voltage VDCS, alternating between the magnitude of the DC input voltage VDCI and zero volts (or the “open” state) is then applied to the primary winding of an isolation transformer 230 to generate a transformed DC switched signal VDCT at the secondary winding. This voltage is then rectified by a rectifier 240 to produce a rectified voltage VR, which may then be passed to a low-pass filter 250 to reduce any AC components of that voltage VR, thus resulting in a final DC output voltage VDCO.
The magnitude of the DC output voltage VDCO is influenced primarily by the operation of the electronic switch circuit 210, which is managed by a control circuit 220. More specifically, the higher the duty cycle of the switches (i.e., the longer they are in the closed or ON state), the higher the DC output voltage VDCO. Thus, the control circuit 220 monitors the DC output voltage VDCO to adjust the duty cycle properly, often through an isolation circuit 260 to maintain galvanic isolation between the AC input power line and the output of the DC-to-DC converter 200.
Many such converters 200 available today can tolerate a wide range of DC power line voltages without the need for multiple taps or a switch. Power supplies that utilize such a converter 200 are termed “universal-input” power supplies.
Another advantage of such a design is that the use of an electronic switch circuit 210 results in the transistors involved transitioning into either the ON (saturated) or OFF (non-conducting) state very quickly, resulting in very little power loss, making the DC-to-DC converter 200 quite efficient. However, this design also results substantially in a square wave for the switched DC voltage VDCS being applied directly to the transformer. Such a signal possesses a strong harmonic content at high frequencies, thus producing a significant amount of injected current into the non-isolated portion of the converter 200. Also, to allow the use of a smaller transformer and other components, the control circuit 220 typically drives the switches at a much higher frequency (e.g., 20 kilohertz (kHz) to a few megahertz (MHz)) than that of an AC power line. Unfortunately, such high frequencies easily couple though the inter-winding capacitance of the transformer 230, therefore augmenting the injected current produced. Additionally, the fact that the magnitude of the switched DC voltage VDCS is relatively high further exacerbates this phenomenon. The resulting square-wave switched DC voltage signal that drives the primary of the transformer produces fast changes in voltages on the transformer and possess harmonic content at high frequencies, unfortunately injecting high common-mode current through the primary to secondary capacitances of the transformer. To eliminate this requires common-mode chokes on the secondary, which depending on the number of desired taps, can be complex and expensive. Capacitors to ground on the secondary would also be necessary to filter the common-mode noise that is generated, thereby compromising isolation.
One attractive approach for combating this is described in U.S. Pat. No. 7,030,689, issued Apr. 18, 2006 to the assignee of the present application. Here, the output of a class-D amplifier is low-pass filtered to drive the primary of the transformer with a sinusoidal waveform. However, readily available class-D amplifiers used in audio applications operate at relatively low frequencies, resulting in larger components, and the frequency of the signal driving the class-D amplifier is typically at least four times the frequency of the waveform on the transformer. Therefore, to increase the frequency of the transformer waveform (and decrease the size of components) entails switching at four times the frequency.
Another approach involves the use of a 60 Hz square-wave DC-DC converter to generate low common-mode current due to the low frequency of operation. The low frequency, however, necessarily results in very large components. Yet another approach is to cancel the common-mode current by creating an equal and opposite current to cancel the common-mode current generated by the circuit. This approach, however, requires calibration and adds parts and complexity.
Due to the drawbacks associated with existing approaches, a need therefore remains for an approved reduction in common-mode current in such applications.